HIV Testing
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Enzyme Linked Immunosorbent Assay
ELISA
First real test to detect HIV (1983)
Was developed 4 years after the first cases of AIDS
Initially designed to screen blood supply
– First month of testing, 1% of blood supply was HIV positive,
– Retesting, 17%
– Confirmatory test, 4%
Is cheap $8 in state labs, $60-75 in private labs
Is a positive predictive value test
Designed to be very sensitive
– Result, gets more false positives
– 90% accuracy rate
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How it Works
Virus protein is first attached to a lab dish
Serum sample is prepared from the patients blood
Is placed in the dish containing bound HIV viral proteins.
If HIV antibodies are in the serum, they will bind to the
dish with HIV proteins
Remove serum and wash dish
Apply a stain that detects antibodies
If positive, dish is stained
If negative, dish is not stained
Is now automated
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False Positives
Fewer than 1% of results are false positives
– 1/1000 = 10/10,000 = 100/100,000
– Say positive when you are not
– Can be devastating
Is one argument against mass screening
Cost as well
2,800,000/280 million US citizens
Why
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Had several children
Rheumatological diseases
Malaria
Alcoholic Hepatitis
Others
Consequence
– Develop another test
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Consequences of a False Positive
Life goes on hold
Depression/Suicide
Family and emotional issues
Others
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False Negatives
Fewer than 1% of results say you are negative
when you are actually infected with HIV
– 1/1000 = 10/10,000 = 100/100,000
– 280,000/280 million people
Many reasons for false negatives
– Window period
– Some people do not develop a immune response to
the virus
Are rare
Still spread the virus
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Other Issues
Some people who are exposed to HIV
may have a successful immune response
and completely eliminate the virus
– VERY VERY Rare
– Most people who are exposed remain infected
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Western Blot
Is a confirmatory test
Also tests for HIV antibodies
Has a lower level of false positives than ELISA
Usually used when a ELISA test has a positive hit
Today, use the combination. If positive in both, then the
sample is considered positive.
99% positive for results when used together.
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How it Works
Uses Gel electrophoresis
HIV is cultured in Human leukemia cells
Cells are broken open
Proteins are placed on a gel that receives an electrical charge
Viral proteins are separated and settle out at different levels
Proteins at different levels are blotted onto strips of nitrocellulose
paper
Serum is placed on the paper
If antibodies are present, they bind to the proteins
Strip is washed
Add a enzyme and color agent
If HIV is present, a color reaction will occur at the site of the HIV
protein.
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Problems
Lack of standardization between NIH,
ARC, DuPont, DOD, and other
organizations
No real agreement about what constitutes
a positive test
Takes longer to run (12-24 hours)
Is more labor intensive
Result – Costs more to run
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Viral Load Test
p24 antigen test (1995)
– Measures the major core protein of HIV (p24)
– Detects the protein by using an antibody
against it.
– Can detect as early as 16 days after infection
Past - Done when full-blown AIDS
developed
Today – Mandatory use at blood and
plasma centers
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Nucleic Acid Testing
NAT
1999
Used to test for HIV-1, and Hepatitis B
and Hepatitis C in blood centers
Use molecular techniques
Generally tests for viral nucleic acids
– Does not indirectly test for antibodies
Has closed the window to 10 days for
detection.
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Problems
Is expensive ($80 minimum)
Requires specially trained lab technicians
Takes 12 hours
Still have a 1% chance for a false negative
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Polymerase Chain Reaction
PCR
Indirectly can measure viral load
Takes any DNA fragment from a single cell
Is exponentially multiplied to an amount large
enough to be measured.
Does not require antibody formation
Very sensitive
– Six molecules/150,000 cells
– One molecule in 10 micro liters of blood
Can be used to detect HIV in people who are
suspected of having the virus but test negative
by other techniques.
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Serologic Testing Algorithm for Recent
HIV Seroconversions
(STARHS)
Uses two different ELISA tests to
determine if a person was recently
infected or has had HIV for a period of
time (6 Months)
– Test 1
Very Sensitive. Detects small numbers of
antibodies
– Test 2
Is less sensitive but picks up large antibodies that
appear 3-5 months after infection.
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Other Tests of Importance
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Rapid Tests
Single Use Diagnosis System (SUDS)
Use reagents to bind to HIV antibodies in
Serum
Has a 99.9% accuracy rate
Is comparable to the newer ELISA tests
Good where people do not return for
results.
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Other Rapid Tests
HEMA - Strip
SERO – Strip
Are used in over 20 countries
Require no additional equipment
Blood or serum is paced at the tip of a strip and
allowed to diffuse along it.
Strip has reagents that react to antibodies
Can be done in 10 minutes
Also has a built in quality control
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Saliva Tests
Example OraSure
Are easier
Less expensive
Noninvasive
May be slightly less accurate
Also has a home version under trials
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Urine tests
Sentinel
Has same advantages as saliva tests
Is less sensitive than blood tests
Testing is done by professional laborites
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Home Testing
Example Home Access Express-HIV Test
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Take a blood sample at home with finger stick
Mail it to a lab
Call in for results
Advantage
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Cost
Most people are the worried well (May be changing)
Disadvantage
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What happens if positive
May need face to face counseling (Hotlines???)
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Who Should Be Tested?
Persons who engage in high-risk behaviors
– Anal sex
IDU
Have sex with people in high-risk groups
Have certain medical symptoms or conditions
Become Pregnant
Had a blood transfusion before 1985
If positive for other STD
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Anonymous vs. Confidential Testing
When testing began was a big debate
– Anonymous was a big mistake
Now, most states require confidential
testing only (Idaho)
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Conclusions and the Future
Have come a long way
Currently have good and reliable tests
Will get better
Cost needs to get lower
Need fast tests that are cheap
Are being done by insurance companies
– Implications are huge for health care and life
insurance companies
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Testing is Not the Answer
It only tells you if you are positive or negative
Only does so if there is enough virus to detect
At this time Prevention is the only answer
In the future, a vaccine or evolution will be the
solution to HIV
– Problem: With new strains and multiple mutations,
does not appear close at this time.
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